Reflective diffraction grating and fabrication method

ABSTRACT

A reflective diffraction grating and a fabrication method are provided. The reflective diffraction grating includes a substrate, a UV-absorbing layer, a grating layer having a binary surface-relief pattern formed therein, and a conforming reflective layer. Advantageously, the UV-absorbing layer absorbs light at a UV recording wavelength to minimize reflection thereof by the substrate during holographic patterning at the UV recording wavelength.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a reflective diffraction grating and,more particularly, to a binary surface-relief reflective diffractiongrating. The present invention also relates to a method of fabricatingsuch a reflective diffraction grating.

BACKGROUND OF THE INVENTION

Reflective diffraction gratings are used to provide wavelengthdispersion in wavelength-selective optical devices, such as wavelengthselective switches (WSSs). Conventional reflective diffraction gratingsinclude metal-coated diffraction gratings and multilayer dielectricgratings.

Although many fabrication methods exist, metal-coated diffractiongratings are most often produced by replication. In this fabricationmethod, a ruled or holographically patterned master is embossed orcopied into a resin grating layer, e.g., formed of epoxy, to form asurface-relief pattern therein. The surface-relief pattern is thencoated with a conforming reflective metal layer to form the metal-coateddiffraction grating. Unfortunately, the resin grating layer is notstable at high temperatures, e.g., greater than 130° C. for an epoxygrating layer. Therefore, care must be taken to use low temperaturesduring the manufacture of devices incorporating such metal-coateddiffraction gratings in order to prevent temperature-induced changes inthe surface-relief pattern. Furthermore, any demolding materials orrelease layers used during the fabrication of such metal-coateddiffraction gratings must be entirely removed from the resin gratinglayer prior to coating with the reflective metal layer in order to avoidreliability issues.

On the other hand, multilayer dielectric gratings are, typically,produced by direct etching. In this method, a surface-relief pattern isetched through an etch mask into a reflective thin-film stack, whichincludes a large number of thin-film dielectric layers. For example,multilayer dielectric gratings including a reflective thin-film stackformed of alternating silica (SiO₂) and tantala (Ta₂O₅) layers aredisclosed in U.S. Pat. No. 8,238,025 to Parriaux, issued on Aug. 7,2012, in U.S. Pat. No. 6,680,799 to Parriaux et al., issued on Jan. 20,2004, in U.S. Pat. No. 5,907,436 to Perry et al., issued on May 25,1999, and in U.S. Patent Application Publication No. 2008/0062523 toRancourt, published on Mar. 13, 2008, which are incorporated herein byreference. Unfortunately, such multilayer dielectric gratings are,generally, difficult and expensive to manufacture because of the largenumber of process steps involved. Furthermore, when the etch mask ispatterned by holography at a recording wavelength, reflection of lightat the recording wavelength by the reflective thin-film stack and/or asubstrate on which the thin-film stack is disposed may lead to undesiredinterference patterns detrimental to the resulting surface-reliefpattern.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to a reflective diffractiongrating comprising: a substrate; an ultraviolet (UV)-absorbing layerdisposed over the substrate for absorbing light at a UV recordingwavelength to minimize reflection thereof by the substrate; a gratinglayer disposed over the absorber layer, having a binary surface-reliefpattern formed therein, wherein the binary surface-relief patternincludes ridges having rectangular or trapezoidal cross-sections,separated by grooves; and a conforming reflective layer disposed overthe binary surface-relief pattern, forming a grating profile.

The present invention also relates to a method of fabricating one ormore reflective diffraction gratings, the method comprising: providing asubstrate; depositing a UV-absorbing layer over the substrate forabsorbing light at a UV recording wavelength to minimize reflectionthereof by the substrate; depositing a grating layer over theUV-absorbing layer; applying a photoresist layer over the grating layer;patterning the photoresist layer by holography at the UV recordingwavelength; etching the grating layer through the patterned photoresistlayer to form a binary surface-relief pattern therein, wherein thebinary surface-relief pattern includes ridges having rectangular ortrapezoidal cross-sections, separated by grooves; removing the patternedphotoresist layer; and depositing a conforming reflective layer over thebinary surface-relief pattern to form a grating profile.

The present invention also relates to a wafer-level method offabricating a plurality of reflective diffraction gratings, whichfurther comprises: testing the grating profile by a wafer-levelcharacterization process to provide a test result; and reworking theconforming reflective layer if the test result is a fail; or singulatingthe substrate to form a plurality of reflective diffraction gratings ifthe test result is a pass.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in greater detail with referenceto the accompanying drawings, which represent exemplary embodimentsthereof, wherein:

FIGS. 1A to 1E are schematic illustrations of steps in a method offabricating a reflective diffraction grating according to the presentinvention;

FIG. 1F is a schematic illustration of a cross-section of a reflectivediffraction grating according to the present invention;

FIG. 1G is a schematic illustration of a cross-section of a portion ofthe reflective diffraction grating of FIG. 1F, showing the gratingprofile;

FIG. 2 is a scanning electron micrograph of a cross-section of a modelgrating profile formed of bulk gold;

FIG. 3 is a flowchart of a wafer-level method of fabricating a pluralityof reflective diffraction gratings according to the present invention;

FIG. 4A is a table of profile parameters for an exemplary reflectivediffraction grating for use in air, having a line frequency of 1150lines/mm;

FIG. 4B is a table of profile parameters for an exemplary reflectivediffraction grating for use in a grism, having a line frequency of 1624lines/mm;

FIG. 4C is a table of profile parameters for an exemplary reflectivediffraction grating for use in a grism, having a line frequency of 1670lines/mm;

FIG. 5 is a plot of reflection intensity and transmission intensityagainst ultraviolet (UV)-absorbing layer thickness for brown tantala andnormal tantala as the UV-absorbing dielectric material; and

FIG. 6 is a scanning electron micrograph of a cross-section of anexemplary reflective diffraction grating.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1F, the present invention provides a reflectivediffraction grating 100, which is fabricated by holographic lithography,i.e., interference lithography, at an ultraviolet (UV) recordingwavelength, as explained in further detail hereinbelow. The UV recordingwavelength is in the UV spectral region, typically, in a wavelengthrange of about 10 nm to about 450 nm, e.g., 193 nm, 248 nm, 365 nm, or436 nm.

The reflective diffraction grating 100 is designed for use, i.e., forreflection and diffraction of light, over an operating wavelength range,which depends on the application. Often, the operating wavelength rangeis in the infrared (IR) spectral region. For example, fortelecommunication applications, the operating wavelength range may be atelecommunication wavelength band, e.g., the conventional band (C band)of about 1520 nm to about 1570 nm, or the long-wavelength band (L band)of about 1555 nm to about 1625 nm. The reflective diffraction gratingmay be designed for use with transverse-magnetic (TM)-polarized, i.e.,p-polarized, light, transverse-electric (TE)-polarized, i.e.,s-polarized, light, or both, i.e., for low polarization-dependent loss.The reflective diffraction grating 100 may be used as a stand-alonecomponent or may be incorporated in another component, e.g., a grism.Often, the reflective diffraction grating 100 is used as part of awavelength-selective device, e.g., a wavelength-selective switch (WSS).

The reflective diffraction grating 100 includes a substrate 110, aUV-absorbing layer 120 disposed over the substrate 110, a grating layer130 disposed over the UV-absorbing layer 120, and a conformingreflective layer 140 disposed over the grating layer 130.

Together, the UV-absorbing layer 120 and the grating layer 130constitute a thin-film stack. Because the thin-film stack is separatefrom the substrate 110, the design of the reflective diffraction grating100 is largely independent of the substrate material. The substrate 110is, typically, formed of a dielectric substrate material, e.g., fusedsilica (SiO₂) or another common glass. Often, a bottom surface 111 ofthe substrate 110 is polished.

The UV-absorbing layer 120 is a continuous unpatterned, i.e., unetched,layer, which is, typically, disposed directly on the substrate 110.Advantageously, the UV-absorbing layer 120 serves to absorb light at theUV recording wavelength to minimize reflection of such light by thesubstrate 110, in particular, by a bottom surface 111 thereof, duringholographic patterning, as explained in further detail hereinbelow.

The UV-absorbing layer 120 is, typically, formed of a UV-absorbingdielectric material, e.g., tantala (Ta₂O₅), niobia (Nb₂O₅), or titania(TiO₂), that absorbs UV light, in particular, light at the UV recordingwavelength. Preferably, the UV-absorbing dielectric material has anextinction coefficient of greater than about 0.025 at the UV recordingwavelength. For example, the UV-absorbing dielectric material may betantala having an extinction coefficient of about 0.028 at 365 nm,referred to as normal tantala. Normal tantala has a substantiallystoichiometric composition, i.e., Ta₂O₅, and, typically, appearssubstantially transparent as a thin film. More preferably, theUV-absorbing dielectric material has an extinction coefficient greaterthan about 0.035 at the UV recording wavelength. For example, theUV-absorbing dielectric material may be tantala having an extinctioncoefficient of about 0.04 at 365 nm, referred to as brown tantala. Browntantala has a composition substoichiometric in oxygen, i.e., Ta₂O_(5-x),and, typically, appears dark brown as a thin film.

The UV-absorbing layer 120 has a thickness large enough to allowsignificant absorption of light at the UV recording wavelength and,accordingly, a significant reduction in the transmission of such light.Typically, the UV-absorbing layer 120 has a thickness of greater thanabout 1.2 μm and less than about 2 μm. Preferably, the UV-absorbinglayer 120 has a thickness of greater than about 1.5 μm.

The grating layer 130 is a patterned layer, which is, typically,disposed directly on the UV-absorbing layer 120. The grating layer 130has a surface-relief pattern 131 formed therein by etching. Typically,only an upper portion of the grating layer 130 is etched, and thegrating layer 130 is continuous. The surface-relief pattern 131 includesridges separated by etched grooves. The surface-relief pattern 131 isbinary, meaning that the ridges are rectangular or trapezoidal incross-section. Typically, the tops of the ridges are substantiallyparallel to the top surface of the substrate 110, and the sidewalls ofthe ridges are substantially perpendicular to the top surface of thesubstrate 110.

The grating layer 130 is, typically, formed of a dielectric gratingmaterial, e.g., silica. Advantageously, as dielectric thin films, thegrating layer 130 and the UV-absorbing layer 120 are, generally,physically stable at temperatures up to a few hundred degrees Celsius.Typically, the grating layer 130 has a thickness of greater than about0.2 μm and less than about 1.5 μm.

The conforming reflective layer 140 is disposed over the surface-reliefpattern 131 of the grating layer 130, i.e., over the tops and sidewallsof the ridges, and over the bottoms of the grooves, so that itsubstantially conforms to the surface-relief pattern 131, forming adiffraction profile 141. With reference also to FIG. 1G, although theconforming reflective layer 140 may be disposed directly on thesurface-relief pattern 131, it is preferable that a thin adhesion layer145 be inserted between the grating layer 130 and the conformingreflective layer 140 to promote adhesion between these layers.Optionally, an additional thin barrier layer 146 may be inserted betweenthe conforming reflective layer 140 and the adhesion layer 145 tosuppress diffusion between these layers, especially at highertemperatures, e.g., greater than about 250° C. Depending on theembodiment, the conforming reflective layer 140 may be conforminglydisposed on the adhesion layer 145 or the barrier layer 146.

The conforming reflective layer 140 is, typically, formed of areflective metal, e.g., gold. The adhesion layer 145 may be formed oftitanium, and the barrier layer 146 may be formed of titanium nitride(TiN). Typically, the conforming reflective layer 140 has a thickness ofgreater than 40 nm on the sidewalls of the ridges, preferably, greaterthan 60 nm. In general, the sidewall thickness is large enough toprevent leakage of light through the sidewalls into the underlyinggrating layer 130, in order to suppress interference between leakedlight and diffracted light. Typically, the adhesion layer 145 and thebarrier layer 146 each have a thickness of a few nanometers.

The conforming reflective layer 140, which substantially conforms to thesurface-relief pattern 131, forms the grating profile 141 of thereflective diffraction grating 100. With reference to FIG. 2, a scanningelectron micrograph of a cross-section of a model grating profile 241formed of bulk gold shows parameters defining the grating profile 241,referred to as profile parameters. The profile parameters include etchdepth d, slant angle a, air-groove width (AGW), and pitch p, whichcorresponds to a sum of the ridge top width and the AGW.

With reference again to FIG. 1F, these profile parameters are selectedon the basis of the application for which the reflective diffractiongrating 100 is used and the desired diffraction efficiency. Typically,the etch depth is between about 0.1 μm and about 1 μm, the slant angleis between about −10° and +10°, the AGW is between about 0.2 μm andabout 0.7 μm, and the pitch is between about 0.5 μm and about 1.0 μm.

With reference to FIG. 1, the present invention also provides a methodof fabricating the reflective diffraction grating 100 by holographiclithography. With particular reference to FIG. 1A, the substrate 110 isprovided, and the thin-film stack is deposited over the substrate 110,typically, by sputtering. Specifically, the UV-absorbing layer 120 isdeposited over the substrate 110, and the grating layer 130 is depositedover the UV-absorbing layer 120.

With particular reference to FIG. 1B, a photoresist layer 150, e.g.,having a thickness between about 0.4 μm and about 0.6 μm, is appliedover the grating layer 130 of the thin-film stack, typically, by spincoating or spray coating. The photoresist layer 150 is then patterned byholography at the UV recording wavelength to form a patternedphotoresist layer 150, illustrated in FIG. 1C.

The photoresist layer 150 is exposed to an interference pattern, whichis recorded in the photoresist layer 150. In general, when a recordingbeam I of light at the UV recording wavelength, e.g., 365 nm, isincident on the photoresist layer 150, e.g., at an angle of incidence ofabout 10° to about 20°, it is partially reflected as reflected beam R₀.If the intensity of the reflected beam R₀ is too high, interferencebetween the incident recording beam I and the reflected beam R₀ can leadto undesired interference patterns, i.e., standing-wave patterns,causing corrugations in the pattern recorded in the photoresist layer150. Such corrugations are detrimental to the resulting surface-reliefpattern 131.

Most of the non-reflected light is transmitted through the photoresistlayer 150 and the thin-film stack, i.e., the grating layer 130 and theUV-absorbing layer 120, resulting in a transmitted beam T₀. When thetransmitted beam T₀ is incident on the bottom surface 111, i.e., thebackside, of the substrate 110, it is partially reflected as abackside-reflected beam T₀′. Such backside reflections are particularlyimportant when the bottom surface 111 of the substrate 110 is polished,as is often required for reflective diffraction gratings used in WSSs.

The inventors of the present invention experimentally determined that,if the intensity of the backside-reflected beam T₀′ is too high,interference between the incident recording beam I and thebackside-reflected beam T₀′ can lead to undesired interference patternscausing corrugations in the pattern recorded in the photoresist layer150. Therefore, the UV-absorbing layer 120 was included in the design ofthe reflective diffraction grating 100 to absorb light at the UVrecording wavelength. Thereby, the intensity of the transmitted beam T₀is decreased, and the intensity of the backside-reflected beam T₀′ iscorrespondingly decreased. In other words, transmission by the thin-filmstack is reduced to minimize backside reflections. Accordingly,undesired interference patterns resulting from backside reflections aresuppressed.

With particular reference to FIG. 1C, the recorded pattern is developedby using a suitable developer or solvent, leaving a patternedphotoresist layer 150, which serves as an etch mask. With particularreference to FIG. 1D, the grating layer 130 is etched, typically, byreactive-ion etching (RIE), through the patterned photoresist layer 150to form the surface-relief pattern 131. Typically, only an upper portionof the grating layer 130 is etched, without etching the UV-absorbinglayer 120. Preferably, the surface-relief pattern 131 is formed to havea base AGW that is larger than a final AGW desired for the gratingprofile 141, allowing a relatively thick conforming reflective layer 140to be deposited. Typically, the base AGW is about 1.5 times larger thanthe final AGW. For example, the base AGW may be about 80% of the pitch,and the final AGW may be about 50% of the pitch. With particularreference to FIG. 1E, the patterned photoresist layer 150 is removed byusing a suitable stripper or solvent to uncover the surface-reliefpattern 131.

With particular reference to FIGS. 1F and 1G, an adhesion layer 145 is,preferably, deposited over the surface-relief pattern 131, and theconforming reflective layer 140 is deposited over the adhesion layer 145to form the grating profile 141. Optionally, a barrier layer 146 may bedeposited over the adhesion layer 145, and the conforming reflectivelayer 140 deposited over the barrier layer 146. Typically, the adhesionlayer 145, the optional barrier layer 146, and the conforming reflectivelayer 140 are deposited by sputtering. The conforming reflective layer140 is deposited to a thickness providing the final AGW desired for thegrating profile 141.

With reference to FIG. 3, the fabrication method can be implemented as awafer-level method 300 of fabricating a plurality of reflectivediffraction gratings. In step 301, a substrate, i.e., a wafer, isprovided. In step 302, a UV-absorbing layer, for absorbing light at a UVrecording wavelength to minimize reflection thereof by the substrate, isdeposited over the substrate. In step 303, a grating layer is depositedover the UV-absorbing layer. In step 304, a photoresist layer is appliedover the grating layer. In step 305, the photoresist layer is patternedby holography at the UV recording wavelength. In step 306, the gratinglayer is etched through the patterned photoresist layer to form a binarysurface-relief pattern therein, which includes rectangular ortrapezoidal ridges separated by grooves. In step 307, the photoresistlayer is removed. In step 308, a conforming reflective layer, typically,together with an adhesion layer, is deposited over the binarysurface-relief pattern to form a grating profile. Preferably, thesurface-relief pattern is characterized, e.g., by physically measuringits AGW using atomic force microscopy (AFM), prior to depositing theconforming reflective layer in step 308, in order to select thedeposition thickness. Further details relating to steps 301 to 308 areprovided in the description of the fabrication method hereinabove.

In the wafer-level fabrication method, an optimal thickness for theconforming reflective layer is determined by iterative deposition andtesting cycles. After the conforming reflective layer is deposited instep 308, the grating profile is tested in step 309 by using awafer-level characterization technique, which may be a physicaltechnique, e.g., AFM, or an optical technique, to provide a pass/failtest result 310 as feedback. If the test result 310 is a fail, theconforming reflective layer is reworked, and the method returns to step308. The conforming reflective layer may be reworked by increasing thelayer thickness or by removing and re-depositing the layer. Layerremoval is carried out by a suitable metal-etch process that ensuresthat the underlying grating layer is not etched or altered in any way.Care must be taken to avoid physically changing the surface-reliefprofile. Steps 308 and 309 are repeated as necessary until an optimalthickness is obtained and the grating profile passes the test.

In an exemplary embodiment, two different tests are performed in step309, allowing coarse and fine adjustment of the thickness of theconforming reflective layer, respectively. During coarse thicknessadjustment, testing the grating profile in step 309 includes physicallymeasuring an AGW of the grating profile, e.g., by AFM, and comparing themeasured AGW to an AGW tolerance. The test result 310 is a pass if themeasured AGW is within the AGW tolerance and a fail if the measured AGWis outside of the AGW tolerance. If the measured AGW is smaller than alow value of the AGW tolerance, deposition of the conforming reflectivelayer may be continued to increase its thickness in step 308. Forexample, the layer thickness may be increased in steps of about 400 Å.Coarse thickness adjustment is continued until the test result 310 is apass at this stage, i.e., the measured AGW is within the AGW tolerance,and a roughly correct design space is reached.

Fine thickness adjustment is then used to fine-tune the thickness of theconforming reflective layer. During fine thickness adjustment, testingthe grating profile in step 309 includes optically measuring adiffraction efficiency (DE) of the grating profile, and comparing themeasured DE to a minimum DE, e.g., about 90%. Preferably, the DE of thegrating profile is measured at several locations on the wafer todetermine a diffraction efficiency uniformity (DEU), which is comparedto a maximum DEU, e.g., about 5%. A diffraction grating test system(DGTS) is, preferably, used to measure the DE and the DEU. The DGTS is astepper-based system which moves the wafer to illuminate, with light ofa desired polarization and wavelength, various grating chip positionsand which then, with properly positioned optics/mirrors, deflects thefirst reflected diffraction order to a detector which measures thediffraction efficiency.

The test result 310 is a pass if the measured DE is greater than theminimum DE and, preferably, if the measured DEU is also less than themaximum DEU. The test result 310 is a fail if the measured DE is lessthan the maximum DE and/or if the measured DEU is greater than themaximum DEU. If the measured DE less than the maximum DE, deposition ofthe conforming reflective layer may be continued to increase itsthickness in step 308. For example, the layer thickness may be increasedin steps of about 100 Å. Fine thickness adjustment is continued untilthe test result 310 is a pass at this stage, i.e., the measured DE isgreater than the minimum DE and, preferably, the measured DEU is lessthan the maximum DEU. If a pass is not reached after this stage, thewafer is reworked by removing the conforming reflective layer.

If the test result 310 is a pass, the substrate is singulated to providea plurality of reflective diffraction gratings in step 311. Typically,the reflective diffraction gratings are then visually inspected.Advantageously, such an iterative wafer-level fabrication method 300,typically, improves device manufacturing yield and reduces defects,e.g., resulting from handling, relative to a chip-level process. Ingeneral, these advantages reduce manufacturing costs.

Some exemplary reflective diffraction gratings for use inwavelength-selective switches (WSSs) are described hereafter to furtherillustrate the present invention. The exemplary reflective diffractiongratings are designed to provide a high diffraction efficiency in thefirst diffraction order in a near-Littrow geometry for TM-polarizedlight, e.g., with a polarization extinction ratio of about 10 dB.Typically, the exemplary reflective diffraction gratings provide adiffraction efficiency of greater than about 90% across thetelecommunication wavelength C band.

In a design method, the reflective metal for the conforming reflectivelayer is first selected, and the profile parameters for the gratingprofile are determined. To ensure high diffraction efficiency, gold wasselected as the reflective metal for the exemplary reflectivediffraction gratings. Profile parameters, i.e., the etch depth d, theslant angle a, and the air-groove width (AGW), were calculated for amodel grating profile of a particular pitch p formed of bulk gold, asshown in FIG. 2, used at a particular angle of incidence (AOI).

FIG. 4A tabulates the profile parameters, the AOI, and their tolerancesfor a first exemplary reflective diffraction grating for use in air asthe incidence medium, having a line frequency of 1150 lines/mm, i.e., apitch of 0.870 μm. FIG. 4B tabulates the profile parameters, the AOI,and their tolerances for a second exemplary reflective diffractiongrating for use in a grism with epoxy as the incidence medium, having aline frequency of 1624 lines/mm, i.e., a pitch of 0.616 μm. FIG. 4Atabulates the profile parameters, the AOI, and their tolerances for athird exemplary reflective diffraction grating for use in a grism withepoxy as the incidence medium, having a line frequency of 1670 lines/mm,i.e., a pitch of 0.599 μm.

Materials and thicknesses are then selected for the thin-film stack,i.e., the grating layer and the UV-absorbing layer. A thickness is alsoselected for the photoresist layer. For the exemplary reflectivediffraction gratings, silica was selected as the grating dielectricmaterial, and a thickness of 0.27 μm was selected for the grating layer.A thickness of 0.53 μm was selected for the photoresist layer.

The UV-absorbing dielectric material and the thickness of theUV-absorbing layer are selected to suppress undesired interferencepatterns during holographic patterning at the UV recording wavelength.For the exemplary reflective diffraction gratings, the UV recordingwavelength was 365 nm, and tantala was selected as the UV-absorbingdielectric material. Both normal tantala, having a refractive index ofabout 2.4 and an extinction coefficient of about 0.028 at 365 nm, andbrown tantala, having a refractive index of about 2.4 and an extinctioncoefficient of about 0.040 at 365 nm, were investigated as UV-absorbingdielectric materials.

The effects of increasing the thickness of the UV-absorbing layer on theintensities of the reflected beam R₀ and the transmitted beam T₀ wereinvestigated for brown tantala and normal tantala as the UV-absorbingdielectric material. In FIG. 5, the reflection intensity for browntantala 501, the reflection intensity for normal tantala 502, thetransmission intensity for brown tantala 503, and the transmissionintensity for normal tantala 504 are plotted against the thickness ofthe UV-absorbing layer, which varied between 0 and 2.0 μm. The recordingbeam I of light at 365 nm was TE-polarized and was incident on thephotoresist layer at an angle of incidence of about 17.2°.

Increasing the thickness of the UV-absorbing layer significantly reducesbackside reflections. The transmission intensity for normal tantala 504decreases from about 80% to about 25% by increasing the thickness of theUV-absorbing layer from 0 to 1.2 μm. The transmission intensity fornormal tantala 504 can be decreased further to about 20% at a thicknessof 1.5 μm, and to about 12% at a thickness of 2.0 μm.

The suppression of backside reflections is even greater when browntantala is used as the UV-absorbing dielectric material. Thetransmission intensity for brown tantala 503 decreases from about 80% toabout 15% when the thickness of the UV-absorbing layer increases from 0to 1.2 μm. The transmission intensity for brown tantala 503 furtherdecreases to about 10% at a thickness of 1.5 μm, and to about 5% at athickness of 2.0 μm.

With respect to the reflection intensities for brown tantala 501 andnormal tantala 502, increasing the thickness of the UV-absorbing layerfrom 0 to 1.2 μm has the effect of reducing the magnitude of theintensity fluctuations, i.e., ripple, from about ±14% to about ±5%. Themagnitude of the intensity fluctuations is slightly smaller for browntantala than for normal tantala. Advantageously, reducing the magnitudeof the intensity fluctuations reduces the sensitivity of the fabricationmethod to variations in the thickness of the UV-absorbing layer.

In this manner, it was found that a UV-absorbing layer formed of browntantala at a thickness of about 1.5 μm provided suitable suppression ofundesired interference patterns and, hence, allowed the fabricationmethod to meet the intended design targets.

Once designed, the exemplary reflective diffraction gratings werefabricated according to the method described hereinabove. A fused silicasubstrate having a thickness of about 3 mm was provided. A UV-absorbinglayer formed of brown tantala was deposited to a thickness of about 1.5μm on the substrate, and a grating layer formed of silica was depositedto a thickness of about 0.27 μm on the UV-absorbing layer.

A photoresist layer was applied on the grating layer to a thickness ofabout 0.53 μm. A pattern was recorded in the photoresist layer byholography at a UV recording wavelength of 365 nm and developed to forma photoresist etch mask. The grating layer was then etched through thephotoresist etch mask to form a binary surface-relief pattern having theetch depth d and the slant angle a tabulated in FIG. 4, but a largerbase AGW corresponding to about 80% of the pitch.

An adhesion layer formed of titanium was deposited on the patternedgrating layer to a thickness of a few nanometers, and a conformingreflective layer formed of gold was deposited on the adhesion layer to athickness of about 100 nm. This relatively large deposition thicknessensured that the grating profile as a whole had good reflectivity, andthat the ridge sidewalls, in particular, had a thickness of greater thanabout 40 nm. By coating the surface-relief pattern with the conformingreflective layer, the AGW of the grating profile was increased to avalue close to the final AGW tabulated in FIG. 4, corresponding to about50% of the pitch.

With reference to FIG. 6, a scanning electron micrograph of across-section of an exemplary reflective diffraction grating 600fabricated according to this fabrication method shows the fused silicasubstrate 610, the UV-absorbing layer 620 formed of brown tantala, thegrating layer 630 formed of silica, and the conforming reflective layer640 formed of gold over the adhesion layer formed of titanium. Inparticular, the enlargement in FIG. 6 shows how the AGW of the gratingprofile 641 was tuned by forming the surface-relief pattern 631 to havea larger base AGW and by depositing the conforming reflective layer 640to a thickness providing the grating profile 641 with the final AGW. Thebase AGW is approximately 1.5 times larger than the final AGW.

Of course, numerous other embodiments may be envisaged without departingfrom the spirit and scope of the invention.

We claim:
 1. A reflective diffraction grating comprising: a substrate;an ultraviolet (UV)-absorbing layer disposed over the substrate forabsorbing light at a UV recording wavelength to minimize reflectionthereof by the substrate; a grating layer disposed over the absorberlayer, having a binary surface-relief pattern formed therein, whereinthe binary surface-relief pattern includes ridges having rectangular ortrapezoidal cross-sections, separated by grooves; and a conformingreflective layer disposed over the binary surface-relief pattern,forming a grating profile.
 2. The reflective diffraction grating ofclaim 1, wherein the UV-absorbing layer has a thickness of greater thanabout 1.2 μm.
 3. The reflective diffraction grating of claim 1, whereinthe UV-absorbing layer is continuous and unpatterned.
 4. The reflectivediffraction grating of claim 1, wherein the UV-absorbing layer is formedof brown tantala.
 5. The reflective diffraction grating of claim 1,wherein the UV-absorbing layer is formed of a UV-absorbing dielectricmaterial having an extinction coefficient of greater than about 0.025 atthe UV recording wavelength.
 6. The reflective diffraction grating ofclaim 1, wherein the reflective diffraction grating was fabricated byholographic lithography at the UV recording wavelength, and wherein theUV recording wavelength is in a wavelength range of about 10 nm to about450 nm.
 7. The reflective diffraction grating of claim 1, wherein thebinary surface-relief pattern has a base air-groove width (AGW) about1.5 times larger than a final AGW of the grating profile.
 8. Thereflective diffraction grating of claim 1, wherein the UV-absorbinglayer is formed of tantala, wherein the grating layer is formed ofsilica, and wherein the conforming reflective layer is formed of gold.9. The reflective diffraction grating of claim 8, further comprising anadhesion layer disposed between the binary surface-relief pattern andthe conforming reflective layer, wherein the adhesion layer is formed oftitanium.
 10. The reflective diffraction grating of claim 9, furthercomprising a barrier layer disposed between the adhesion layer and theconforming reflective layer, wherein the barrier layer is formed oftitanium nitride.
 11. A method of fabricating one or more reflectivediffraction gratings, the method comprising: providing a substrate;depositing an ultraviolet (UV)-absorbing layer over the substrate forabsorbing light at a UV recording wavelength to minimize reflectionthereof by the substrate; depositing a grating layer over theUV-absorbing layer; applying a photoresist layer over the grating layer;patterning the photoresist layer by holography at the UV recordingwavelength; etching the grating layer through the patterned photoresistlayer to form a binary surface-relief pattern therein, wherein thebinary surface-relief pattern includes ridges having rectangular ortrapezoidal cross-sections, separated by grooves; removing the patternedphotoresist layer; and depositing a conforming reflective layer over thebinary surface-relief pattern to form a grating profile.
 12. The methodof claim 11, wherein the UV-absorbing layer is deposited to a thicknessof greater than about 1.2 μm.
 13. The method of claim 11, wherein thegrating layer is etched by etching an upper portion of the gratinglayer, without etching the UV-absorbing layer.
 14. The method of claim11, wherein the binary surface-relief pattern is formed to have a baseair-groove width (AGW), wherein the conforming reflective metal layer isdeposited to a thickness providing the grating profile with a final AGW,and wherein the base AGW is about 1.5 times larger than the final AGW.15. The method of claim 11, further comprising depositing an adhesionlayer over the binary surface-relief pattern, wherein the conformingreflective layer is deposited over the adhesion layer.
 16. The method ofclaim 15, further comprising depositing a barrier layer over theadhesion layer, wherein the conforming reflective layer is depositedover the barrier layer.
 17. The method of claim 11, further comprising:testing the grating profile by using a wafer-level characterizationtechnique to provide a test result; and reworking the conformingreflective layer if the test result is a fail; or singulating thesubstrate to form a plurality of reflective diffraction gratings if thetest result is a pass.
 18. The method of claim 17, wherein the substrateis reworked by removing and re-depositing the conforming reflectinglayer.
 19. The method of claim 17, wherein the substrate is reworked bycontinuing deposition of the conforming reflecting layer.
 20. The methodof claim 17, wherein testing the grating profile includes measuring anAGW of the grating profile, a diffraction efficiency of the gratingprofile, and/or a diffraction efficiency uniformity of the gratingprofile.